US20220075106A1 - Optical filtering device - Google Patents

Optical filtering device Download PDF

Info

Publication number
US20220075106A1
US20220075106A1 US17/447,225 US202117447225A US2022075106A1 US 20220075106 A1 US20220075106 A1 US 20220075106A1 US 202117447225 A US202117447225 A US 202117447225A US 2022075106 A1 US2022075106 A1 US 2022075106A1
Authority
US
United States
Prior art keywords
slits
reflective elements
wavelength
filter
refractive index
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
US17/447,225
Other languages
English (en)
Inventor
Jérôme LE PERCHEC
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Original Assignee
Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Commissariat a lEnergie Atomique et aux Energies Alternatives CEA filed Critical Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Assigned to Commissariat à l'Energie Atomique et aux Energies Alternatives reassignment Commissariat à l'Energie Atomique et aux Energies Alternatives ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: Le Perchec, Jérôme
Publication of US20220075106A1 publication Critical patent/US20220075106A1/en
Pending legal-status Critical Current

Links

Images

Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/18—Diffraction gratings
    • G02B5/1809—Diffraction gratings with pitch less than or comparable to the wavelength
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/20—Filters
    • G02B5/26—Reflecting filters
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/18—Diffraction gratings
    • G02B5/1814—Diffraction gratings structurally combined with one or more further optical elements, e.g. lenses, mirrors, prisms or other diffraction gratings
    • G02B5/1819—Plural gratings positioned on the same surface, e.g. array of gratings
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/18—Diffraction gratings
    • G02B5/1847—Manufacturing methods
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/18—Diffraction gratings
    • G02B5/1866—Transmission gratings characterised by their structure, e.g. step profile, contours of substrate or grooves, pitch variations, materials
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/20—Filters
    • G02B5/204—Filters in which spectral selection is performed by means of a conductive grid or array, e.g. frequency selective surfaces
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/0121—Operation of devices; Circuit arrangements, not otherwise provided for in this subclass
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/20—Filters
    • G02B5/208—Filters for use with infrared or ultraviolet radiation, e.g. for separating visible light from infrared and/or ultraviolet radiation
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F2203/00—Function characteristic
    • G02F2203/05—Function characteristic wavelength dependent
    • G02F2203/055—Function characteristic wavelength dependent wavelength filtering

Definitions

  • the present invention relates to an optical filtering device having a transmission response with a maximum at a wavelength ⁇ 0 .
  • the invention relates also to a tunable optical filtering unit incorporating this optical filtering device.
  • Such filters are notably applicable in the field of visible and infrared photodetection, more particularly intended for the spectral detection of gases, and for visible and infrared colour imaging.
  • micro- or nano-structured optical filtering devices are generally preferred for their great compactness which allows them to be incorporated in photodetectors.
  • the filtering device has a narrow transmission passband around the wavelength ⁇ 0 and a significant rejection ratio outside of this passband.
  • This narrow passband is also called “transmission peak” in this text.
  • the transmission ratio is equal to the ratio between the quantity of incident electromagnetic radiation on the optical filtering device and the quantity of electromagnetic radiation having completely passed through the device.
  • the transmission ratio is equal to 1 or to 100%, at a wavelength ⁇ , when the device is completely transparent to that wavelength ⁇ .
  • the rejection ratio is the inverse of the transmission ratio.
  • the rejection ratio is equal to 1 or 100%, at a wavelength ⁇ , when the device is completely opaque to the electromagnetic radiation at that wavelength ⁇ .
  • This known optical filtering device comprises reflective elements deposited on a face of a half-wave plate. These reflective elements define a sub-wavelength periodic grating of parallel through slits. The assembly formed by this grating and the waveguide plate constitutes a resonator that behaves as a passband filter around the wavelength ⁇ 0 .
  • the maximum amplitude of the transmission ratio at the wavelength ⁇ 0 increases with the width of the slits.
  • FIG. 6 of the patent U.S. Pat. No. 8,937,277B1 shows, the greater the width of the slits the more degraded the out-of-passband rejection ratio.
  • the greater the width of the slits the greater the width of the passband of the optical filtering device.
  • the selectivity of this optical filtering device degrades as the width of the slits increases. Because of that, in the known optical filtering devices, it is accepted that the best trade-off is obtained when the width of the slits is systematically chosen to be less than P/3, where P is the periodicity of the slits. However, even by observing this constraint, it is difficult to obtain a high transmission ratio while retaining a high rejection ratio and a high selectivity.
  • the invention aims to propose an optical filtering device which allows the transmission ratio to be increased without degrading, even enhancing, the rejection ratio and the selectivity.
  • the subject of the invention is an optical filtering device.
  • a subject of the invention is a tunable optical filtering unit which implements the above optical filtering device.
  • FIG. 1 is a schematic view, in vertical cross-section, of a first embodiment of an optical filtering device
  • FIG. 3 is a plot of the transmission response of the device of FIG. 1 and of the filter of FIG. 2 ;
  • FIG. 4 is a plot of the transmission response of the device of FIG. 1 for different thicknesses of an intermediate layer
  • FIG. 5 is a flow diagram of a first method for manufacturing the device of FIG. 1 ;
  • FIGS. 6 to 9 are schematic illustrations, in vertical cross-section, of different manufacturing states obtained in the implementation of the method of FIG. 5 ;
  • FIG. 10 is a flow diagram of a second method for manufacturing the device of FIG. 1 ;
  • FIG. 15 is a view, in vertical cross section, of a second embodiment of an optical filtering device
  • FIG. 16 is a plot of the transmission response of the devices of FIGS. 1 and 15 ;
  • FIG. 17 is a view, in vertical cross-section, of a third embodiment of an optical filtering device
  • FIG. 18 is a schematic illustration of a tunable optical filtering unit
  • FIG. 19 is a schematic illustration, in vertical cross-section, of a detector incorporating a grating of optical filtering devices.
  • FIG. 1 represents an optical filtering device 2 .
  • the transmission response of the device 2 exhibits a single transmission peak corresponding to a high transmission ratio.
  • the transmission ratio is said to be “high” if it exceeds 80% or 85%.
  • This high transmission ratio is obtained for a wavelength ⁇ 0 .
  • the device 2 On either side of the transmission peak, the device 2 exhibits a high rejection ratio, that is to say a rejection ratio greater than 95% or 99%, even greater than 99.9%.
  • This high rejection ratio is observed over ranges of wavelengths situated to the right and to the left of the transmission peak. These ranges each extend over a width greater than ⁇ 0 /2.
  • the transmission peak of the device 2 is in the mid-infrared, that is to say between a wavelength of 3 ⁇ m and a wavelength of 5 ⁇ m.
  • the wavelength ⁇ 0 is equal to 4 ⁇ m.
  • the device 2 comprises two one-way optical filters 4 and 6 arranged one on top of the other in a vertical direction and separated from one another by an intermediate layer 8 .
  • the orientation in space is identified with respect to an orthogonal reference frame XYZ.
  • the direction Z is the vertical direction.
  • the directions X and Y are contained in a horizontal plane.
  • the direction X is parallel to the cutting plane of the device 2 .
  • the terms such as “high”, “low”, “above”, “below”, “higher”, “lower” and similar are defined with respect to the direction Z.
  • the direction of the incident electromagnetic radiation is represented by an arrow I in FIG. 1 .
  • the incident electromagnetic radiation is propagated vertically downwards.
  • the incident electromagnetic radiation has a polarization TM (Transverse Magnetic) with the electrical field in the plane XZ.
  • the filter 4 extends primarily horizontally. In the vertical direction, it extends from a lower adjacent plane P 4inf to a higher adjacent plane P 4sup .
  • the planes P 4inf and P 4sup are horizontal.
  • the planes P 4inf and P 4sup are situated at the interface between the filter 4 and external media of low refractive indices situated, respectively, below the plane P 4inf and above the plane P 4sup .
  • the medium of low refractive index situated under the plane P 4inf is the medium 10 .
  • the medium of low refractive index situated above the plane P 4sup is the layer 8 .
  • the filter 6 extends from a lower adjacent plane P 6inf to a higher adjacent plane P 6sup .
  • the planes P 6inf and P 6sup are, here, respectively symmetrical to the planes P 4sup and P 4inf with respect to a median horizontal plane P m .
  • the plane P m is the median plane of the layer 8 . It is situated at mid-height of the layer 8 .
  • the planes P 6inf and P 6sup are situated at the interface between the filter 6 and external media of low refractive indices.
  • the medium of low refractive index situated under the plane P 6inf is the layer 8 .
  • the medium of low refractive index situated above the plane P 6sup is a medium 20 .
  • the filter 4 comprises, in succession, directly deposited one on top of the other: —a plate 12 arranged on a top horizontal face of the medium 10 , this plate having a refractive index n 12 at the wavelength ⁇ 0 and a thickness h 12 in the vertical direction, and—reflective elements 14 deposited on a horizontal top face of the plate 12 situated on the side opposite the medium 10 .
  • the medium 20 is a gaseous medium whose refractive index n 20 at the wavelength ⁇ 0 is less than the index n 22 and, preferably, less than 3 n 22 /5 or less than 1.5.
  • the medium 20 is air. Air is particularly advantageous because it has a refractive index close to 1.
  • the reflective elements 24 are identical to the reflective elements 14 . Furthermore, they are arranged with respect to one another in the same way as the reflective elements 14 . In this embodiment, the reflective elements 24 are symmetrical to the reflective elements 14 with respect to the plane P m .
  • the layer 8 is produced in a dielectric material having a refractive index n 8 at the wavelength ⁇ 0 .
  • the refractive index n 8 is low, that is to say less than 3 n 12 /5 and, preferably, less than 1.5.
  • the thickness of the layer 8 is denoted h 8 .
  • the layer 8 extends from the plane P 4sup to the plane P 6inf .
  • the thickness h 8 is therefore equal to the distance which separates the planes P 4sup and P 6inf.
  • This thickness h 8 is chosen to create a moderate coupling between the evanescent fields generated by the filters 4 and 6 . Such a choice for the thickness h 8 is explained in detail later.
  • the plate 12 is transparent to the wavelength ⁇ 0 and forms a waveguide and, preferably, a half-wave plate.
  • the plate 12 is produced in a dielectric material of strong refractive index.
  • an element produced in a material X means that this material X represents at least 90% or 95% of the mass of the element X.
  • a “strong refractive index” means that the index n 12 is greater than the indices n 10 and n 8 and, preferably, greater than 5 n 10 /3 and than 5 n 8 /3.
  • the reflective elements 14 are produced in a material whose electrical permittivity is negative at the wavelength ⁇ 0 .
  • the reflective elements 14 are produced in metal and, in this example, in aluminium.
  • the reflective elements 14 define a periodic grating of slits 30 intended to be passed through by the incident electromagnetic radiation.
  • the number of slits 30 is greater than ⁇ 0 / ⁇ , where ⁇ is the mid-height width of the transmission peak at the wavelength ⁇ 0 .
  • the grating of slits 30 is formed on the top face of the plate 12 .
  • the slits 30 emerge on the top face of the plate 12 .
  • the characteristics of the slits 30 are chosen such that the grating exhibits a transmission selectivity around the wavelength ⁇ 0 .
  • the thickness h met of the reflective elements 14 in the vertical direction is low, that is to say less than 50 nm or 100 nm.
  • the thickness h met is also greater than the skin thickness p met ( ⁇ 0 ), at the wavelength ⁇ 0 , of the material used to produce the reflective elements 14 .
  • the thickness h met is between p met ( ⁇ 0 ) and 5 p met ( ⁇ 0 ).
  • the width w of the slits 30 is chosen to obtain a maximum transmission ratio greater than 90%. To this end, the width w is generally chosen between P/3 and 2P/3.
  • the reflective elements 14 are arranged in a medium of refractive index less than 1.5.
  • this medium is the layer 8 .
  • the propagation mode of the electromagnetic wave which resonates in each of the plates 12 and 22 is associated with an evanescent field which decreases exponentially in the intermediate layer 8 .
  • / ⁇ ), where: —A e is the amplitude of the evanescent field at a distance z from the plate 12 inside the layer 8 , — ⁇ is a coefficient defined by the following relationship: 1/ ⁇ (2 ⁇ n 8 / ⁇ 0 )(( ⁇ 0 /(n 8 P)) 2 ⁇ 1) 0.5 , where P is the period of the grating of slits, —the symbol “exp” is the exponential function.
  • the thickness h 8 is chosen between 1.25 ⁇ and 2.75 ⁇ or between 1.5 ⁇ and 2.5 ⁇ or between 1.9 ⁇ and 2.1 ⁇ . In this embodiment, the thickness h 8 is equal to 2 ⁇ . The choice of this thickness h 8 is justified by the experimental results below.
  • FIG. 3 represents, on one and the same graph, the transmission response of the isolated filter 4 (curve 50 ) and the transmission response of the device 2 (curve 52 ).
  • the x axis indicates the wavelength in nanometres and the y axis indicates the corresponding transmission ratio expressed between 0 and 1, where the value 1 corresponds to a transmission ratio of 100%.
  • the coefficient ⁇ is equal to 277 nm.
  • the thickness h 8 must be chosen between 346 nm and 761 nm.
  • the thickness h 8 is equal to 550 nm.
  • the maximum amplitude of the transmission peak of the insulated filter 4 is very high and reaches 94% for the wavelength equal to 4.07 ⁇ m.
  • the maximum amplitude of the transmission ratio of the device 2 is also very high since it reaches 92% for the wavelength equal to 3.98 ⁇ m.
  • the maximum amplitude of the transmission peak of the device 2 is better than that which would be obtained in the absence of moderate coupling of the evanescent fields of the filters 4 and 6 .
  • the rejection ratio to the right and left of the transmission peak of the insulated filter 4 is less than 90%. Contrary to the insulated filter 4 , the rejection ratio to the right and to the left of the transmission peak of the device 2 is very high and remains greater than 99%. For example, the rejection ratio is equal to 87.4% at the wavelength equal to 3.5 ⁇ m for the insulated filter 4 whereas it is equal to 99.4% for the device 2 .
  • the device 2 exhibits both a very high transmission ratio and a very high rejection ratio outside of the transmission peak.
  • the resonance wavelength of the device 2 is also a little offset with respect to that of the insulated filter 4 .
  • the curves 60 and 62 correspond to a strong coupling of the evanescent fields. It can be seen that such a strong coupling splits into at least two the transmission peak which degrades the selectivity of the device 2 and also the rejection ratio outside of the transmission peak.
  • the thickness h 8 is chosen greater than 3 ⁇ , the coupling between the evanescent fields generated is very weak such that the improvement in the transmission ratio with respect to the case where the insulated filters 4 and 6 would not be coupled to one another via their evanescent fields is negligible or non-existent. This is illustrated by the curve 68 of FIG. 4 . Furthermore, in the absence of coupling between the evanescent fields of the filters 4 and 6 , a spurious transmission peak appears. In the graph of FIG. 4 , this spurious transmission peak appears in the curve 68 around the wavelength equal to 2.6 ⁇ m.
  • the thickness of the layers 74 and 76 is equal to the thickness h met .
  • the top face of the layer 76 is directly exposed to the outside.
  • the medium 10 and the layer 8 are made of amorphous silicon oxide 10 whose refractive index is approximately 1.4 for a wavelength ⁇ 0 equal to 4 ⁇ m.
  • the plate 12 is produced in amorphous silicon whose refractive index is approximately 3.84 for the wavelength ⁇ 0 .
  • a step 80 the stack 72 is etched to produce the reflective elements 14 and 24 in the layers, respectively, 74 and 76 .
  • vertical trenches 82 FIG. 7
  • Each of these trenches 82 passes successively through the layer 76 , the layer 8 and the layer 74 .
  • a coating 94 ( FIG. 9 ) is deposited on the top face of the plate 24 .
  • the coating 94 is a protective layer or an anti-reflection layer.
  • This method begins with the supply, in a step 100 , of a stack identical to the stack 72 except that the layers 8 and 76 are omitted.
  • a step 102 the metallization layer 74 is etched to form the reflective elements 14 on the top face of the plate 12 ( FIG. 11 ).
  • the intermediate layer 8 is deposited on the reflective elements 14 ( FIG. 12 ). Given that the reflective elements 14 are separated from one another by slits, the face on which the layer 8 is deposited is not flat. The result thereof is that nor is the top face of the layer 8 after its deposition flat. It exhibits unevennesses with respect to each reflective element 14 .
  • the reflective elements 24 are produced on the top face of the layer 8 .
  • the top face of the layer 8 is not polished before producing the reflective elements 24 .
  • the reflective elements 14 are produced by the deposition of a metallization layer on the top face of the layer 8 then by etching of this metallization layer.
  • the plate 22 is deposited on the reflective elements 24 ( FIG. 14 ).
  • the plate 22 is deposited on a face which is not flat, its top face comprises unevennesses with respect to each of the reflective elements 24 .
  • the plate 22 is not exactly symmetrical to the plate 12 with respect to the plate P m .
  • the optical filtering device obtained exhibits a high transmission ratio and high rejection ratios on either side of the transmission peak.
  • FIG. 15 represents an optical filtering device 120 identical to the device 2 except that the filter 6 is replaced by an optical filter 122 .
  • the filter 122 is identical to the filter 6 except that the order in which the plate 22 and the reflective elements 24 are stacked on one another is reversed. In other words, in this embodiment, the bottom face of the plate 22 is flush with the plane P 6inf and the reflective elements 24 are arranged on the top face of the plate 22 in contact with the medium 20 .
  • FIG. 16 represents the transmission response of the device 2 (curve 130 ) and of the device 120 (curve 132 ).
  • the scale of they axis is logarithmic. It can be observed that the maximum amplitude of the transmission peak of the device 2 is slightly greater than the maximum amplitude of the transmission peak of the device 120 . Here, a difference of approximately 1.6% is observed.
  • the device 2 exhibits a rejection ratio slightly greater than that of the device 120 .
  • the performance levels of the device 120 are a little lower than those of the device 2 , the transmission and rejection ratios obtained with the device 120 remain much better than those obtained with the insulated filter 4 or with a superposition of the filters 4 and 6 but without moderate coupling of their evanescent fields.
  • the graph of FIG. 16 also comprises a curve 134 which corresponds to an optical filtering device identical to the device 2 except that the reflective elements 24 are offset, for example in the direction X, by a pitch equal to P/4.
  • the performance levels of this embodiment are a little lower than those of the device 2 but the transmission and rejection ratios obtained remain much better than those obtained with the insulated filter 4 or with the superposition of the filters 4 and 6 but without moderate coupling of their evanescent fields.
  • FIG. 17 represents an optical filtering device 140 .
  • the device 140 is identical to the device 120 except that the medium 10 is deposited on a support 142 which has a high refractive index n 142 .
  • the index n 142 is greater than 1.5 or than 2.
  • the support 142 is produced in silicon or in germanium.
  • the medium 10 forms, preferably, a quarter-wave plate.
  • the thickness h 10 of the medium 10 is chosen between 0.9 M ⁇ 0 /(4 n 10 ) and 1.1 M ⁇ 0 /(4 n 10 ), where M is an odd integer number chosen in advance.
  • the thickness h 10 is equal to M ⁇ 0 /(4 n 10 ).
  • FIG. 18 represents a tunable optical filtering unit 150 .
  • the unit 150 comprises an optical filtering device 152 and a voltage source 154 .
  • the device 152 is, for example, identical to the device 2 except that it comprises two connection terminals 156 and 158 . All the reflective elements 14 are electrically connected to the terminal 156 . All the reflective elements 24 are electrically connected to the terminal 158 . The reflective elements 14 and the terminal 156 are electrically insulated from the reflective elements 24 and from the terminal 158 .
  • the reflective elements 24 are connected to the terminal 158 in a way similar to that described for the reflective elements 14 . Thus, when a potential difference is generated between the terminals 156 and 158 , an electrical field is generated in the intermediate layer 8 .
  • the intermediate layer 8 is produced in a dielectric material which is, in addition, an electro-optical material, that is to say a material in which it is possible to modify the refractive index under the effect of a steady-state or low-frequency electrical field.
  • an electro-optical material that is to say a material in which it is possible to modify the refractive index under the effect of a steady-state or low-frequency electrical field.
  • one of the best known nonlinear effects is the Pockels effect in which the change of index is directly proportional to the electrical field passing through it.
  • the coefficient R can be of the order of 100 pm/V, even more.
  • such a polymer material is PMMA (polymethyl methacrylate).
  • the source 154 is electrically connected between the terminals 156 and 158 . It is capable of generating a potential difference between these terminals 156 and 158 .
  • the source 154 comprises a module 170 for setting the potential difference between the terminals 156 and 158 .
  • the potential difference between the terminals 156 and 158 using the module 170 it is possible to modify certain characteristics of the device 152 such as, for example, the position of its transmission peak or the modulation of its amplitude. For example, with a height h 8 equal to 500 nm, a potential difference U equal to 50 Vdc and a coefficient R equal to 100 pm/V, the variation ⁇ n 8 of refractive index n 8 is equal to 0.1.
  • FIG. 19 represents a multi-pixel detector 180 .
  • This detector 180 comprises a matrix of incident electromagnetic radiation detection cells.
  • each of these cells is formed by a detecting diode produced in a semiconductor substrate 186 .
  • FIG. 19 only two detecting diodes 182 and 184 are represented. The junctions of these diodes 182 , 184 are produced in this substrate 186 .
  • optical filtering device is placed in front of each of these diodes.
  • this filtering device is, for example, identical to the device 140 except that the support 142 is replaced by the substrate 186 .
  • the optical filtering devices situated in front of the diodes 182 , 184 respectively bear the numeric references 140 a and 140 b.
  • the substrate 186 and the medium 10 are common to all the optical filtering devices.
  • the substrate 186 and the medium 10 therefore extend horizontally and continually in front of each of the detecting diodes.
  • a trench 190 is hollowed out along the periphery of each of the devices 140 a and 140 b in order to minimize the interferences between these different optical 10 filtering devices.
  • the trench 190 separates the plates 12 , 22 and the intermediate layer 8 of each of the filtering devices from the plates 12 , 22 and from the intermediate layer 8 of the filtering devices situated immediately alongside.
  • the characteristics of the device 140 a can be different from the characteristics of the device 140 b.
  • the period P of the device 140 a is different from the period P of the device 140 b.
  • FIG. 20 represents another multi-pixel detector 190 identical to the detector 180 except that each detection cell is formed by a bolometer of micrometric size.
  • bolometers 192 and 194 are represented.
  • Each bolometer 192 , 194 makes it possible to measure the energy of the incident electromagnetic radiation in the infrared range.
  • Such bolometers are known and only the main production details of these bolometers are reviewed here.
  • the bolometer 192 comprises a thermo-resistive membrane 196 supported by pillars 200 , 202 , on top of a reflective substrate 204 .
  • the distance which separates the substrate 204 from the membrane 196 is equal or approximately equal to a quarter of the wavelength to be absorbed.
  • each bolometer is encapsulated in a cavity hollowed out to insulate it from the outside medium, as notably explained in the patent EP1243903B1.
  • the walls of this cavity are, for example, produced in amorphous silicon.
  • a filtering device is produced above each bolometer.
  • the filtering devices situated above, respectively, the bolometers 192 and 194 respectively bear the numeric references 208 and 210 .
  • the filtering devices 208 and 210 are identical, for example, to the device 2 .
  • the medium 10 of each of these filtering devices corresponds to the medium filling the interior of the hollowed-out cavity in which the bolometer is situated.
  • the plate 12 forms the top wall of the hollowed-out cavity.
  • the thickness h 10 of the medium 10 is equal to the distance separating the plate 12 from the membrane 196 of the bolometer. As described previously, preferably, the thickness h 10 is between 0.9 M ⁇ 0 /(4 n 10 ) and 1.1 M ⁇ 0 /(4 n 10 ), where M is an odd integer number chosen in advance. Typically, in the case of a bolometer, the medium 10 is a vacuum.
  • FIG. 21 represents another possible embodiment of a two-dimensional filter 230 that can be used in place of the filters 4 and 6 .
  • the filter 230 is identical to the filter 4 except that the reflective elements 14 are replaced by reflective elements 232 .
  • the reflective elements 232 are identical to the reflective elements 14 except that they each have a rectangular horizontal section and therefore form blocks aligned one behind the other in the directions X and Y.
  • the reflective elements 232 are arranged with respect to one another in such a way as to define two periodic gratings of slits 234 and 236 of respective widths w 234 and w 236 and of respective periods P 234 and P 236 . These two gratings are mutually orthogonal. In this case, by taking the periods P 234 and P 236 as equal and by taking the widths w 234 and w 236 as equal, the filter 230 is insensitive to the polarization of the incident electromagnetic radiation.
  • the rejection ratio on either side of the transmission peak, is improved.
  • the widths w 234 and w 236 are chosen to be relatively narrow, namely to lie rather between P 234 /5 and P 234 /3.
  • the thickness of the reflective elements 232 is preferably around 50 nm and less than 100 nm.
  • the excitation of these secondary peaks is promoted with an excessive width w 234 .
  • the width w 234 must be adapted to obtain an acceptable trade-off between the rejection ratio desired over the entire spectral range of study and the desired transmission peak amplitude.
  • an optical filtering device produced with two-dimensional filters makes it possible to obtain rejection ratios better than those obtained with only the filter 230 .
  • the orthogonal projection of a reflective element on a horizontal plane is a parallelogram and not necessarily a rectangular parallelogram.
  • the horizontal section of the reflective elements is not necessarily square. In this latter case, the periods P 234 and P 236 are not equal.
  • the periods P 234 and P 236 are sufficiently close for the intersection between these two ranges not to be empty.
  • the reflective elements 24 are not necessarily symmetrical to the reflective elements 14 .
  • they are offset in any horizontal direction by a distance less than the period P.
  • the reflective elements 24 are offset, in a horizontal direction, by a distance equal to P/2 or P/4.
  • the elements 24 are arranged with respect to one another like the elements 14 but are no longer symmetrical to the elements 14 with respect to the plane P m .
  • the reflective elements are deposited on each of the faces of the plate 12 .
  • the plate 12 comprises a first grating of slits on its bottom face and a second grating of slits on its top face.
  • these first and second gratings of slits are structurally identical.
  • they are each identical to the grating of slits defined by the reflective elements 14 or 232 .
  • reflective elements are deposited on each of the faces of the plate 22 and the gratings of slits formed on each of the faces of the plate 22 are identical, respectively, to the first and second gratings of slits.
  • the reflective elements can be produced in materials other than aluminium. For example, they are produced in silver. Aluminium and silver are good choices, in particular, for the production of an optical filtering device whose transmission peak is in the visible range, that is to say typically between 400 nm and 800 nm. However, other metals can also be used such as gold, copper, platinum or an alloy thereof.
  • the metals can be replaced by any material exhibiting reflective properties similar to those of the metals in the range of wavelengths targeted by the filtering.
  • a material that has a relative permittivity with high imaginary part namely of the order of 10 or more, that is to say a strongly conductive or strongly absorbent material, can be used.
  • silicon can be used in the ultraviolet (10 nm to 380 nm) and strongly N-doped silicon can be used in the infrared.
  • An ionic crystal exhibiting a negative permittivity can also be used, such as SiC, in the far infrared (towards 10-12 ⁇ m) for example.
  • the reflective elements forming the gratings of slits are deposited directly on the plates 12 and 22 .
  • a thin layer is provided, of the order of a tenth of the thickness of the plate, and of refractive index lower than that of the plate, so as to modify the refractive index on contact with the plate.
  • This additional thin layer is, for example, a graded index layer and can moreover fulfil another function, such as a securing, passivation or protection function.
  • the width w can also be chosen less than P/3 to have a highly selective filter exhibiting a rejection ratio better than that obtained with a filter constructed in accordance with the teaching of the application U.S. Pat. No. 8,937,277B2. In this case, the transmission ratio is not necessarily improved.
  • the plates 12 and 22 can be produced in GaP, AlAs, GaSb, or PbTe which exhibit high refractive indices in the optical range.
  • GaP is a good choice if the transmission peak of the device is in the visible range.
  • a silicon-rich (x>1) silicon nitride Si x N 1-x less costly, can be used in the visible.
  • the material used can be one of the following materials: MgF 2 , BaF 2 , CaF 2 , LiF.
  • MgF 2 is a good choice if the transmission peak of the device is in the visible range.
  • the layer 8 and the media 10 and 20 can also be produced by a stacking of several materials of low refractive indices.
  • the layer 8 and the medium 10 are not necessarily produced in a material in the solid state. They can also be produced using a material in the gaseous state, such as air or liquid. In the case of a material in gaseous or liquid state, shims produced in a material in solid state are interposed between the filters 4 and 6 to obtain the desired thickness of the intermediate layer 8 .
  • the use of a material in gaseous state for the production of the layer 8 is advantageous notably if the transmission peak is situated in the visible wavelength range.
  • the intermediate layer 8 is a polarizable conductive transparent oxide (for example ITO), connected electrically to a third terminal, and the reflective elements 14 and 24 are situated, respectively, in the planes P 4inf and P 6sup of the stack and each connected to the terminals 156 and 158 .
  • ITO polarizable conductive transparent oxide
  • an electrical field between the intermediate layer 8 and the reflective elements can be applied to modify the refractive index of the waveguide plates 12 and 22 .
  • the medium 20 is not necessarily a gaseous medium.
  • the medium 20 can also be a material in solid or liquid state whose refractive index satisfies the same conditions as those stated for the medium 20 .
  • the layer 8 and the media 10 and 20 are non-metallic.
  • the reflective elements 14 are electrically connected to the terminal 156 via a film that is electrically conductive and transparent to the incident electromagnetic radiation.
  • This film is, for example, deposited over the entire top surface of the reflective elements 14 and of the slits 30 .
  • this film can be produced in ITO (indium tin oxide) and have a thickness of approximately 50 nm.
  • the electrical field in the layer 8 is generated using conductive plates situated, respectively, above the plate 22 and below the plate 12 and no longer using reflective elements 14 and 24 . In this case, it is these conductive plates which are connected to the terminals 156 , 158 .
  • the layer 8 of the device 152 can be produced in other electro-optical materials.
  • the layer 8 can be produced based on nematic liquid crystals, whose effective refractive index, of the order of 1.5 to 2, can be modified by application of a polarization voltage applied between the terminals 156 and 158 .
  • the modification of the refractive index n 8 is caused by a change of orientation of the molecules of the layer 8 . That has been demonstrated, for example, in the article “ Polarization - independent actively tunable colour generation on imprinted plasmonic surfaces ” by D. Franklin et al., Nat Commun 6, 7337 (2015).
  • the optical filtering device comprises more than two identical optical filters stacked one on top of the other in the direction Z.
  • the intermediate layer which separates two optical filters that are immediately consecutive in the direction Z is produced as described for the layer 8 .
  • the device 2 described above with a grating of strips extending in the direction Y does not exhibit a transmission peak of great amplitude for a TE (transverse electrical) polarized incident electromagnetic radiation whose electrical field is parallel to the direction Y.
  • a polarization conversion system which converts the non-polarized or TE polarized incident electromagnetic radiation into an electromagnetic radiation exhibiting just a TM polarization.
  • the article “ Efficient and monolithic polarization conversion system based on a polarization grating ”, by J. Kim et al, Applied Optics 51, 4852 (2012)) describes such a microstructured and compact system with a conversion efficiency close to 90%.
  • the electromagnetic radiation for which the polarization has been converted then passes through the device 2 .
  • the association of the conversion system and of the device 2 makes it possible to maximize the power of the flux transmitted by the filtering device.
  • the teaching given in this text can also be implemented by using the filters 4 and 6 whose transmission peaks are not high.
  • the device 2 exhibits a transmission peak higher than the square of the amplitude of the peaks of the filters 4 and 6 without in any way being necessarily greater than 85% or 90%.
  • the rejection ratio of the device 2 is not necessarily greater than 90%, but remains in all cases greater than that of the filters 4 and 6 taken alone.
  • each of the optical filters conforms to practically all the teachings of patent U.S. Pat. No. 8,937,277B1 makes it possible to obtain an optical filtering device which retains the advantages of the optical filters described in that patent.
  • the optical filtering device is compact. It is tunable in an extended range of wavelengths, ranging from 250 nm to a hundred or so micrometres. It has a selective transmission passband, namely a mid-height width of the transmission peak less than 10% of the wavelength for which the peak is maximum.
  • the device exhibits both a higher transmission ratio and a higher rejection ratio over a spectral band around the transmission peak of a width at least equal to half the wavelength for which the peak is maximum.
  • the rejection ratio is typically greater than 90%.
  • Producing the reflective elements in silver or in aluminium makes it possible to minimize the optical losses by absorption and improve the transmission ratio notably when the wavelength ⁇ 0 is in the visible range.
  • Producing the intermediate layer in a material which is also electro-optical makes it possible to dynamically modify the characteristics of the optical filtering device.
  • the reflective elements are rectangular makes it possible to obtain a grating of slits parallel to the direction X and a grating of slits parallel to the direction Y.
  • the device makes it possible to transmit both the component of the electromagnetic radiation polarized parallel to the slits, and also the orthogonally polarized component.

Landscapes

  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Nonlinear Science (AREA)
  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Optical Filters (AREA)
  • Light Guides In General And Applications Therefor (AREA)
  • Mechanical Light Control Or Optical Switches (AREA)
US17/447,225 2020-09-10 2021-09-09 Optical filtering device Pending US20220075106A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FR2009172 2020-09-10
FR2009172A FR3113954B1 (fr) 2020-09-10 2020-09-10 Dispositif de filtrage optique

Publications (1)

Publication Number Publication Date
US20220075106A1 true US20220075106A1 (en) 2022-03-10

Family

ID=74045620

Family Applications (1)

Application Number Title Priority Date Filing Date
US17/447,225 Pending US20220075106A1 (en) 2020-09-10 2021-09-09 Optical filtering device

Country Status (3)

Country Link
US (1) US20220075106A1 (de)
EP (1) EP3968064B1 (de)
FR (1) FR3113954B1 (de)

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN113848605A (zh) * 2021-11-16 2021-12-28 电子科技大学 基于介质超表面的窄带光偏振转换器

Family Cites Families (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2822541B1 (fr) 2001-03-21 2003-10-03 Commissariat Energie Atomique Procedes et dispositifs de fabrication de detecteurs de rayonnement
FR2900279B1 (fr) * 2006-04-19 2008-06-06 Commissariat Energie Atomique Filtre spectral micro-structure et capteur d'images
FR2959021B1 (fr) 2010-04-15 2012-07-27 Commissariat Energie Atomique Filtre optique mono ou multi-frequentiel et detecteur comportant un tel filtre
FR3007148B1 (fr) * 2013-06-17 2016-11-25 Centre Nat De La Rech Scient - Cnrs Element de filtrage optique angulaire pour le filtrage angulaire a selectivite angulaire controlee
FR3065132B1 (fr) * 2017-04-06 2019-06-28 Office National D'etudes Et De Recherches Aerospatiales (Onera) Dispositif et procede d'imagerie multispectrale dans l'infrarouge

Also Published As

Publication number Publication date
FR3113954B1 (fr) 2023-03-03
EP3968064A1 (de) 2022-03-16
EP3968064B1 (de) 2024-04-17
FR3113954A1 (fr) 2022-03-11

Similar Documents

Publication Publication Date Title
Li et al. Aluminium nitride integrated photonics: a review
US6532326B1 (en) Transverse-longitudinal integrated resonator
Sapriel et al. Tunable acoustooptic filters and equalizers for WDM applications
EP0347563A2 (de) Wellenlängenmultiplexermodul
Gupta et al. Development and characterization of two-transducer imaging acousto-optic tunable filters with extended tuning range
US8937277B2 (en) Mono- or multifrequency optical filter, and detector comprising such a filter
US20090220184A1 (en) Electro-Optical Modulator Structure
US20110110628A1 (en) Semiconductor light-receiving element, optical communication device, optical interconnect module, and photoelectric conversion method
KR20140031899A (ko) 최소의 각 의존성을 갖는 표시 장치들 및 이미징을 위한 스펙트럼 필터링
US7068903B2 (en) Optical element using one-dimensional photonic crystal and spectroscopic device using the same
Parmar et al. Graphene-silicon hybrid chirped-superstructure bragg gratings for far infrared frequency
JP4954975B2 (ja) 周期構造体及び周期構造の作製方法並びに応用製品
US20220075106A1 (en) Optical filtering device
US8829633B2 (en) Self-aligned semiconductor ridges in metallic slits as a platform for planar tunable nanoscale resonant photodetectors
CN111141385A (zh) 窄带透射滤波器及片上光谱分析与成像系统
Huang et al. Ultrasensitive and tunable multi-narrowband metamaterial absorber
Santos Collinear light modulation by surface acoustic waves in laterally structured semiconductors
Awad Nano-plasmonic Bundt Optenna for broadband polarization-insensitive and enhanced infrared detection
Gu et al. Flat-top filter using slanted guided-mode resonance gratings with bound states in the continuum
Dainese et al. Directional coupler wavelength selective filter based on dispersive Bragg reflection waveguide
Bauser et al. Amorphous silicon carbide high contrast gratings as highly efficient spectrally selective visible reflectors
Wood et al. Low-voltage, broadband graphene-coated Bragg mirror electro-optic modulator at telecom wavelengths
KR101891913B1 (ko) 투과형 구조색 필터 및 이의 제조방법
Zihao et al. All-optical multichannel switch and slow light based on dynamically tunable
CN220381310U (zh) 一种偏振特性可调的高对比度光栅偏振器

Legal Events

Date Code Title Description
AS Assignment

Owner name: COMMISSARIAT A L'ENERGIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES, FRANCE

Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:LE PERCHEC, JEROME;REEL/FRAME:057427/0367

Effective date: 20210830

STPP Information on status: patent application and granting procedure in general

Free format text: DOCKETED NEW CASE - READY FOR EXAMINATION